The present invention relates to burners generally, and more particularly to a low NO.sub.x burner having enhanced flame stability and a construction that minimizes vibration generation and accompanying furnace rumble (low-frequency loud noise).
Generally, NO.sub.x emissions rise exponentially with combustion temperature. These emissions typically are reduced by lowering combustion temperatures. In some cases this is accomplished by combusting the fuel with an increased amount of excess air (lean mixture).
One example of a system using excess air to reduce NO.sub.x emissions is disclosed in the article "The Development of a Natural Gas-Fired Combustor for Direct-Air" from the 1992 International Gas Research Conference. In this burner system, the fuel and gas are premixed and then injected in the combustion chamber. The air-fuel mixture is adjusted to provide whatever amount of excess air is desired to lower the temperature so that NO.sub.x emissions are minimized. However, one of the drawbacks of this system is that there remains the danger of explosions upstream from the combustion chamber, for example, in the burner.
In U.S. Pat. No. 5,102,329, a low NO.sub.x burner is disclosed, in which mixing of fuel gas and combustion air to the extent necessary for combustion in the burner is precluded. In this burner, fuel tubes or spuds are arranged over slots in a burner plate to discharge fuel gas therethrough at high velocities. Combustion air also is discharged from the burner through these slots. Although some mixing of fuel gas and combustion air (controlled exclusively by fuel gas jet entrainment of the combustion air) occurs along the boundary line between each cone-shaped fuel gas jet and the air, the space volume where this mixing occurs is negligible. In addition, the flow pattern in this area has a velocity component in the downstream direction that many times exceeds the propagation velocity of the flame. Accordingly, any flame flashback from the combustion chamber is precluded.
Although the above systems advantageously reduce NO.sub.x emissions, and in the latter case, minimize the possibility of flame flashback, they are subject to combustion or air flow driven pulsation of the flame front, which causes strong vibration and rumbling in the furnace. In burners generally, the combustion amplifies pulsations which typically occur at a frequency of about 8-200 Hz due to the particular characteristics of the air supply fan or duct work, for example, or resonance modes of a furnace. It has been found that when heat of combustion is applied rapidly and uniformly to the flow of fuel and air downstream of the burner in the area of combustion, these pulsations can be amplified more easily. As a result, the flame front oscillates toward and away from the burner plate at a frequency determined by the system. This leads to vibrations, and causes resonance of the hardware of the furnace, known as rumbling. These vibrations, and resonance problems are of particular concern in large combustion devices.
Another way to reduce flame temperature, and consequently NO.sub.x emissions, is to enhance entrainment of relatively cold oxygen deficient gases from the furnace volume into the combustion space by using the kinetic energy of the air and fuel flows. One example of this is the "transjet" burner manufactured by Hague International. The drawbacks of this design are its inability to effectively control NO.sub.x emissions with an increase of excess air, large size for a given heat input, and high air pressure requirement. Expensive heat and corrosion resistant materials also are required with this system.